That movement is detectable in data from the Wilkinson Microwave Anisotropy Probe (WMAP), which NASA says "reveals conditions as they existed in the early universe by measuring the properties of the cosmic microwave background radiation over the full sky"—radiation thought to have been released about 380,000 years after the birth of the universe.

Hot gas in galaxy clusters warms the microwave background radiation, and "a very tiny component of this temperature fluctuation also contains in itself information about cluster velocity," Kashlinsky said.

If a cluster were moving faster or slower than the universe's background radiation, you'd expect to see the background heated slightly in that region of the universe—the result of a sort of electron-scattering "friction" between the cluster's hot gas and particles in the background radiation.

Because these fluctuations are so faint, the team studied more than 700 galaxy clusters.

The researchers had expected to find that, the farther away clusters are, the slower they appear to be moving.

Instead, Kashlinsky said, "We found a great surprise."

The clusters were all moving at the same speed—nearly 2 million miles (3.2 million kilometers) an hour —and in a single direction.

Though this dark flow was detected only in galaxy clusters, it should apply to every structure in the known universe, Kashlinsky said.

Explaining the Unexplainable

To explain the unexplainable flow, the team turned to the longstanding theory that rapid inflation just after the big bang had pushed chunks of matter beyond the known universe.

The extra-universal matter's extreme mass means it "could still pull—tug on—the matter in our universe, causing this flow of galaxies across our observable horizon," said Kashlinsky, whose team's study appeared in the October 20 issue of the Astrophysical Journal Letters.

"Strong Doubts"

Not everyone is ready to rewrite physics just yet.

Astrophysicist Hume Feldman of the University of Kansas has detected a similar, but weaker, flow.

He said the Kashlinsky team's study is "very interesting, very intriguing, [but] a lot more work needs to be done.

"It's suggestive that something's going on, but what exactly is going on? It basically tells us to investigate," he said.

David Spergel, an astrophysicist at Princeton University, echoed the sentiment.

"Until these results are reanalyzed by another group, I have strong doubts about the validity of the conclusions of this paper," he wrote in an email.

He added that, if the result does hold up, "it would have an important implication for our understanding of cosmology."

Study leader Kashlinsky agrees many questions remain unanswered. For starters: What exactly are these things that are apparently tugging our universe?

"They could be anything. As bizarre as you could imagine—some warped space-time," Kashlinsky said.